Effect of Quaternary Ammonium Salt on Shear Bond Strength of Orthodontic Brackets to Enamel.

Objectives
This study sought to assess the effect of quaternary ammonium salt (QAS) on shear bond strength of orthodontic brackets to enamel.


Materials and Methods
In this in vitro experimental study, 0, 10, 20 and 30% concentrations of QAS were added to Transbond XT primer. Brackets were bonded to 60 premolar teeth using the afore-mentioned adhesive mixtures, and the shear bond strength of the four groups (n=15) was measured using a universal testing machine. After debonding, the adhesive remnant index (ARI) score was determined under a stereomicroscope. Data were analyzed using one-way ANOVA.


Results
The mean and standard deviation of shear bond strength of the control and 10%, 20% and 30% groups were 23.54±6.31, 21.81±2.82, 20.83±8.35 and 22.91±5.66 MPa, respectively. No significant difference was noted in shear bond strength of the groups (P=0.83). Study groups were not different in terms of ARI scores (P=0.80).


Conclusions
The results showed that addition of QAS to Transbond XT primer had no adverse effect on shear bond strength of orthodontic brackets.


INTRODUCTION
Prevention of enamel demineralization around orthodontic brackets is a challenge in orthodontic treatment [1]. Orthodontic brackets and appliances complicate oral hygiene practice for patients and enhance the accumulation of dental plaque on tooth surfaces [2]. Bracket bonding is routinely performed during fixed orthodontic treatment. Despite numerous advantages, bracket bonding has a risk of debonding and plaque accumulation around brackets [3]. Bracket debonding may occur during orthodontic treatment and increases the duration of treatment and cost and necessitates additional patient visits [4]. Composite resins used for adhesive bracket bonding have a polymer matrix and can provide a suitable environment for proliferation of aerobic and anaerobic microorganisms [5]. Therefore, researchers are working on new formulations of bonding agents and composite resins with antibacterial activity to minimize the occurrence of white spot lesions. Several antibacterial agents have been added to orthodontic adhesives and composite resins to confer antibacterial activity. The antibacterial activity of quaternary ammonium salts (QASs) has been previously confirmed [6,7]. QAS is present in the formulation of Clearfil Protect Bond. QASs bond to bacterial cell membrane and cause its lysis and death [8].  [11]. The teeth were then mounted in molds measuring 2.5x2.5 cm. The internal surface of the mold was coated with petroleum jelly and the teeth were fixed using 16x22 inch rectangular stainless steel ligature wire. Each tooth was positioned at the center of the mold and the rectangular wire was fixed to the mold using sticky wax so that the teeth remained fixed when applying acrylic resin. Autopolymerizing acrylic resin was applied to the mold and the teeth were embedded in acrylic to the level of their cementoenamel junction. After polymerization of acrylic resin, the teeth in acrylic blocks were separated from the mold (Fig. 1). The shear bond strength test was performed in Tehran University Dental Research Center. Universal testing machine (Zwick Roell, Ulm, Germany) was used for shear bond strength testing. The teeth were placed in the machine such that the bracket base was parallel to the load application vector. Load was applied in occlusogingival direction at a crosshead speed of 0.5 mm/minute to the brackettooth interface (Fig. 2). Load at debonding was recorded in Newtons (N) and converted to Megapascals (MPa) by dividing the load in Newtons by the bracket base surface area in square-millimeters (mm 2 ).
The ARI score was determined as follows: Score 1. All adhesive remaining on the enamel surface Score 2. 90% of adhesive remaining on the enamel surface Score 3. 10-90% of adhesive remaining on the enamel surface Score 4. Less than 10% of adhesive remaining on the enamel surface Score 5. No adhesive remaining on the enamel surface [12] RESULTS

Results of shear bond strength:
The mean bond strength values of the four groups are presented in Table 2. As shown, the highest bond strength was noted in the control group and the lowest in 20% group. According to ANOVA, the difference in shear bond strength was not significant among the four groups (P=0.83). Results of ARI score: Table 3 shows the results of ARI scores. The Kruskal Wallis test showed no significant difference among the groups in terms of ARI scores (P=0.80).

DISCUSSION
This study sought to assess the effect of addition of QAS to Transbond XT primer on shear bond strength of orthodontic brackets to the enamel. No significant difference was noted in shear bond strength of the groups. Study groups were not different in terms of ARI scores. Orthodontic brackets enhance plaque accumulation in the oral cavity [13] and white spot lesions develop in 50 to 75% of patients under fixed orthodontic treatment [14]. Addition of antibacterial agents to composite resins and adhesives has been suggested as an effective approach for prevention of enamel demineralization during orthodontic treatment [15]. QASs have been suggested for this purpose due to their confirmed antibacterial activity. However, the effect of addition of this compound on physical and mechanical properties of adhesives and composite resin should also be evaluated. A previous study showed that addition of QAS to composite resin caused no change in shear bond strength of brackets after one month [8]. This finding was in agreement with our results since we found no significant difference in shear bond strength of the groups containing QAS and the control group. No standard method exists for assessment of bracket bond strength to tooth structure because in the clinical setting, loads applied to brackets are complex and include a mixture of shear and tensile loads, which cannot be well simulated in vitro. However, shear bond strength test is often performed for this purpose [15] and was adopted in the current study as well. The results showed that by an increase in concentration of QAS, no significant reduction occurred in shear bond strength. Several factors affect the bracket bond strength to tooth structure including the type of adhesive, type of bracket, clinician's expertise and patient's cooperation [16].  [9] evaluated the addition of QAS and silver nanoparticles to orthodontic primer and concluded that they had no adverse effect on bond strength. In this study, the effect of QAS on immediate bond strength was evaluated. Future studies are recommended to compare immediate and long-term bond strength after addition of QAS to orthodontic primer. Our study showed no significant difference in ARI scores of the four groups, which was in line with the results of a previous study [22]. However, our study had an in vitro design, and could not well simulate the clinical setting. In the oral environment, a combination of tensile, shear and torsional loads are applied and significant thermal changes, humidity, acidity and microbial plaque further complicate the situation, which cannot be ideally simulated in vitro. Thus, generalization of in vitro results to the clinical setting must be done with caution [23].

CONCLUSION
Within the limitations of this study, the results showed that QAS had no adverse effect on bond strength of orthodontic brackets to enamel.